Motor vehicles typically have temperature-controlled measuring resistors to measure the mass flow rate of intake air entering the internal combustion engine. Typically, the measuring resistor is a hot wire or heated film. The intake air flows over the heated wire or film, which is coupled to an electrical bridge circuit. The operating temperature of the measuring resistor is greater than the intake air temperature, and is maintained at a constant level by controlling the level of electric current flowing therethrough.
The level of electric current necessary to heat the measuring resistor corresponds to the mass flow rate of air entering the engine. Typically, the electric current for heating the measuring resistor is converted into a voltage signal. The value of the voltage signal is, accordingly, indicative of the mass flow rate of air entering the engine. Based on the voltage signal, an engine control unit adjusts engine input variables to efficiently operate the engine.
Any contamination on the surface of the hot wire or film decreases the measurement sensitivity thereof. Therefore, after the engine is turned off, the wire or film is heated to a red-hot temperature by increasing the level of electric current flowing therethrough. Any contaminants on the wire or film are thus burned away.
One problem with known methods and apparatus for measuring the mass flow rate of intake air entering an engine, is that an optimum degree of measurement sensitivity cannot be attained over the entire range of air mass flow rates encountered during operation of the engine. The operating temperature of the measuring resistor is typically maintained at one level. Ordinarily, that level does not provide an optimum degree of measurement sensitivity over the entire range of intake air mass flow rates encountered in the operation of the engine. For example, the degree of measurement sensitivity provided during full load operation of the engine is ordinarily insufficient to accurately measure the air mass flow rates at low load operational modes. As a result, the measurements of the air mass flow rates are typically not as accurate as desired over the entire operational range of an engine.
It is an object of the present invention, therefore, to provide a method and apparatus for measuring the mass flow rate of air entering an internal combustion engine that provides optimum degrees of measurement sensitivity over the entire range of intake air mass flow rates encountered during operation of the engine.